Area Forecast Discussion
Issued by NWS Medford, OR

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FXUS66 KMFR 260406
AFDMFR

Area Forecast Discussion
National Weather Service Medford OR
906 PM PDT Tue Jul 25 2017

.DISCUSSION...There were widespread thunderstorms east of the
Cascades and over the Marble Mountains of western Siskiyou
County. Some of these storms did produce moderate to heavy rain
and there were reports of widespread pea sized hail. One
thunderstorms is trying to drift westward over the Cascades toward
Prospect but it looks unlikely to drift into the Rogue Valley.
Will let today`s Red Flag Warning expire at 11 PM as planned.

The closed low pressure will start to move inland on Wednesday.
The Rogue Valley appears to be out of the thunderstorm threats but
northern California will be under the threat again. /FB

&&

.AVIATION...25/18Z TAF CYCLE...Along the coast and over the coastal
waters...Isolated MVFR/IFR cigs will increase to areas of MVFR/IFR
cigs this evening and spread into the coastal valleys and lower
Umpqua valley tonight.  The lower conditions will clear back to the
coast by late Wednesday morning. Over the remainder of the
area...VFR conditions will prevail...but there will be isolated to
scattered thunderstorm activity, peaking in the afternoon and
evening. Some higher terrain will be obscured near thunderstorms.


&&

.MARINE...Updated 230 PM PDT Tuesday 25 July 2017...Conditions
will remain hazardous to small craft for all areas tonight with
winds diminishing north of Cape Blanco. Seas will subside below
small craft advisory levels north of Cape Blanco on Wednesday, but
the thermal trough will remain in place through the remainder of
this week and into this weekend, with small craft advisory
conditions persisting south of Cape Blanco. -Spilde


&&

.PREV DISCUSSION... /Issued 345 PM PDT Tue Jul 25 2017/

SHORT TERM...Today through Wednesday night...
Showers and thunderstorms have blossomed this afternoon generally
from Crater Lake National Park north and northeastward, from Winter
Rim eastward, and across Northern California. The activity, thus
far, has been very slow moving. Despite this slow movement, storm
cores have been very narrow, so the footprint of lightning appears
to be substantially outside of storm cores, for the most part.
Through this evening we expect storm motion to slowly increase.
However, a complicating factor is counteracting increasing low
level northwest winds coming from the West Side and heading
eastward this evening. This is likely to undercut development that
will be generally drifting in from the east. We expect some cells
to form on the Siskiyous and in the Upper Klamath Basin this
evening, attempting to drift onto the West Side this evening and
overnight. Given the current wind flow and the undercutting
northwesterly low level winds, expect that more southeastern edges
of the West Side such as eastern Douglas County, the Ashland area,
and the Applegate are areas to keep a close eye on for thunderstorm
activity this evening and overnight. We have reached convective
temperature here at the airport at 98 degrees now, which greatly
increases the chances that storms will roll into the valley this
evening. With precipitable water values up around and inch, some
areas will see locally heavy rainfall up to about and inch from
these storms. However, due to the nature of the monsoon, some areas
will receive no rain and, possibly, cloud to ground lightning.

Tonight into Wednesday there is expected to be a trough lingering
across the middle of the East Side to the Cascades, possibly into
Jackson County, so we have left some threat of thunder in overnight
into Wednesday morning. Wednesday afternoon and evening showers and
storms are expected to form in similar areas to today, but are
likely to begin further west and then kick out to the east faster.
The core of the pesky upper level low will be drifting across
northern California tomorrow afternoon and evening, with 2 of 3
primary models drifting it across Modoc County. Thus, we expect
another round of significant lightning before things calm down on
Thursday. BTL

LONG TERM...Thursday onward...
The upper low/trough that brought isolated to scattered
thunderstorms over the last four days will finally move east of
our area on Thursday. In it`s wake will be a southwest flow. A
weak upper trough will approach southwest Oregon Thursday
afternoon bringing building cumulus in parts of western Siskiyou
County and along and east of the Cascades. However instability is
marginal at best and moisture is lacking. The ECMWF shows some
convective feedback in portions of western Siskiyou County and
Cascades, but suspect this may be overdone due to insufficient
moisture and marginal instability parameters. For now we`ll keep
any mention of thunder our of the forecast, but if future model
runs show stronger support for storms, they may need to be added.

The above mention upper trough will be slow to move Friday, but like
Thursday, moisture may be lacking and instability marginal at best.
After Friday, the models differ with the evolution of the weak upper
trough. The GFS brings the upper trough just west of the bay area in
California Saturday and could tap into some monsoonal moisture that
could get drawn up into the eastside along with weak shortwaves.
This scenario could result in at least a slight chance of
thunderstorms. In contrast the ECMWF has the trough weakening and
progressing inland which would yield dry conditions. Either way it`s
going to be hot with temperatures approaching triple digits for some
west side locations and low to mid 90s for the eastside. For now did
not mention any chance of thunder, but this will have to be
monitored closely.

The ridge is expected to retrograde west near or over our area
Sunday into early next week with continued dry and hot weather. The
GFS has the ridge portioned further east which could tap into
additional monsoonal moisture which could move into portions of the
eastside. The ECMWF has the ridge centered in northern California
and western Nevada. Right now, we think the the chance for
thunderstorms are low during this time period, but this could
change. Temperatures will remain hot with near triple digit readings
in some westside valley locations and low to mid 90s on the eastside.
-Petrucelli

AVIATION...25/18Z TAF CYCLE...Along the coast and over the coastal
waters...Isolated MVFR/IFR cigs will increase to areas of MVFR/IFR
cigs this evening and spread into the coastal valleys and lower
Umpqua valley tonight.  The lower conditions will clear back to the
coast by late Wednesday morning. Over the remainder of the
area...VFR conditions will prevail...but there will be isolated to
scattered thunderstorm activity, peaking in the afternoon and
evening. Some higher terrain will be obscured near thunderstorms.

MARINE...Updated 230 PM PDT Tuesday 25 July 2017...Conditions
will remain hazardous to small craft for all areas tonight with
winds diminishing north of Cape Blanco. Seas will subside below
small craft advisory levels north of Cape Blanco on Wednesday, but
the thermal trough will remain in place through the remainder of
this week and into this weekend, with small craft advisory
conditions persisting south of Cape Blanco. -Spilde

FIRE WEATHER...Updated 200 PM Tuesday 25 July 2017...Scattered
thunderstorms are forecast over the majority of the forecast
area this afternoon and evening, focused primarily from the
Siskiyous and Cascades eastward. However, thunderstorms do have
the potential to move off the terrain and over the West Side
valleys. Some storms will produce wetting rain because of the
increasing moisture and expected slow storm motion, but abundant
lightning will still cause fire starts.

Tonight there is continued potential for thunderstorms, and it looks
like the biggest threat will be east of the Cascades. It now looks
like some areas will have scattered coverage, so the Red Flag
Warning will be extended through the night for those areas.

On Wednesday the upper low that is now along the Northern California
coastline will move eastward, so the area of potential thunderstorm
activity will also move eastward.  Therefore, the Red Flag Warning
for Wednesday will not include some west side areas that are in
today`s warning.

The thunderstorm threat will diminish Wednesday night as the upper
low moves out to the east of the area and southwest to west
flow aloft develops over the area. This will likely lead to
breezy west winds during the second half the week.

&&

.MFR WATCHES/WARNINGS/ADVISORIES...
OR...Red Flag Warning until 11 PM PDT Wednesday for
     ORZ617-621-623>625.
     Red Flag Warning until 11 PM PDT this evening for ORZ620-622.

CA...Red Flag Warning until 11 PM PDT Wednesday for
     CAZ280-281-284-285.

Pacific Coastal Waters...Small Craft Advisory until 11 PM PDT Friday for PZZ356-376.
     Small Craft Advisory for hazardous seas until 11 AM PDT
     Wednesday for PZZ350-356-370-376.
     Hazardous Seas Warning until 11 PM PDT this evening for
     PZZ356-376.

$$

FJB/FJB/MAS



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